The mammalian musculature provides mechanical force for locomotion. This system undergoes remodeling throughout lifetime and displays remarkable ability to regenerate from reservoirs of muscle stem cells. However, the muscle regenerative capacity declines with age. This proposal focuses on defining the origin(s) and roles of muscle stem cells during the life span of a mouse. We propose to use a knock-in allele replacing Pax7 with the Cre-ERT2 fusion gene (the Pax7CE allele) for tamoxifen (tmx) inducible cell lineage tracing in following aims: Aim 1: Pax7 lineage during embryogenesis: We will use the Pax7CE allele to determine: 1) When and where do Pax7+ embryonic cells become fate-restricted to the muscle lineage? 2) When do Pax7+ embryonic cells take on a satellite cell fate? 3) Do embryonically derived Pax7 descendant cells give rise to adult satellite cells capable of muscle regeneration? Aim 2: Pax7 lineage in adult muscles: We will further ask: 1) After forced exercise, do Pax7+ cells become activated and fuse into existing myofibers? 2) When Pax7+ cells are genetically ablated by diptheria toxin (DTA) in adulthood, can muscle maintain homeostasis in mass and/or regenerate after acute injury? Aim 3: Pax7 lineage in aged muscles: Aged (> 2 year old for a mouse) muscles are poor in regeneration and accumulate fibrosis and adipocytes. We design experiments to ask: 1) How quiescent are adult satellite cells? 2) Are aged satellite cells a different population of stem cells from those of young adults? 3) Do aged Pax7+ satellite cells trans-fate into fibroblasts or adipocytes after injury?